Lithium-ion batteries (LIBs) are widely used in modern society. Since LIBs represent a substantial fraction and sometimes even majority of the device cost, extending the lifetime of LIBs and understanding their degradation mechanisms draw an increasing attention. Graphite, commonly used as the negative electrode in LIBs, suffers from two main degradation mechanisms: loss of active material and loss of lithium-ion inventory. Formation of the so-called solid electrolyte interphase (SEI) layer is known as the key factor for functioning of the anode. On the one hand, its growth covering the electrode surface slows down Li-ion intercalation. On the other hand, the SEI layer both protects the graphite particles from exfoliation upon intercalation and the electrolyte from decomposition by preventing organic molecules from entering graphite. Due to surface defects of the graphite particles used in common LIBs, it is challenging to observe the SEI layer formation. Therefore, highly oriented pyrolytic graphite (HOPG), which has a perfectly smooth surface, is commonly used in SEI formation experiments.

In this work, we have applied electrochemical analysis, as well as morphology and structure characterization to investigate processes involved into the formation of SEI layer.

A three-electrode electrochemical cell setup was installed in a glove box under inert argon atmosphere. HOPG was used as the working electrode and lithium metal was employed as both the reference electrode (RE) and counter electrode (CE). The electrolyte was composed of 1M lithium hexafluorophosphate (LiPF₆) dissolved in 1:1 v/v mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Cyclic voltammetry (CV) was performed in voltage ranges of 2, 1.5, and 0.5 V versus Li/Li⁺. canning Electron Microscopy (Zeiss XB1540) and Atomic Force Microscopy (Ntegra Aura) were used to characterize the morphology and topography of the HOPG electrode after CV. X-ray Photoelectron Spectroscopy (XPS, Phoibos 150 1D-DLD, XR 50, Specs) was employed to measure the elemental composition and structure at the HOPG surface.

When the cycling was performed between 3 – 2 V, small particles were observed on the surface of the HOPG, which can be assigned to the deposition of electrolyte. Homogeneous microspheres started to appear when the voltage reached below 1.5 V, which can be attributed to the formation of SEI particles. As the CV range increased, which also meant an increase in the depth of discharge (DOD), the density of microspheres on the surface increased as well. The diameter of the microspheres was found to be around 80 – 100 nm at a low lithiation level, and as the HOPG electrode became fully lithiated, the size of the particles slightly increased. This finding was supported by XPS measurements where the integral of the Li_xPF_y and Li_xPF_yO_y peak, which is related to the main constituents of SEI layer, was found to be the largest (57.68% of the full F 1s spectra peak area) in the 3 – 0 V cycle range, while a lower peak (54.56%) was observed in the 3 – 0.5 V range (in the attached Figure).

These results provide an indication of how the charging method affects the capacity fade and lifetime of the LIBs. The stage from 0.5 to 0 V on HOPG is approximately equivalent to 80-100% SOC for a full cell. As was found, in this stage, the SEI film grows thicker, thereby aggravating the aging of the cell. The lifetime of LIBs can therefore be extended if cycling at high SOC is avoided. Based on this, our future work will focus on the development of an optimized multi-step fast-charging method, which can reduce degradation and extend the lifetime of commercial lithium-ion batteries.